RAPID PROTOTYPING AND EXPERIMENTAL DESIGN

As an aviator, I’ve always found interest in aerospace applications of strain gages. Strain gage use in testing aircraft goes way back to the 1940’s when experimental stress analysis was used to validate designs which had to be lighter and thus, faster and able to carry more fuel or payload. In fact, in many instances a design was strengthened by removing material, thanks to the information provided by strain gages. In addition to measuring strain for stress analysis, strain gages have been used to adapt aircraft structural and control components so that force and load measurements can be obtained. Applications for strain gages are as infinite as the human imagination and new applications continue to evolve.

In this article, appearing in the August 2018 issue of Kitplanes magazine, Eric Stewart describes a cutting-edge application of strain gages by TECAT. With wireless technology provided by TECAT, and hands-on training by the experts at Micro-Measurements, Eric now has the ability to read torque and horsepower right in the cockpit of racing aircraft.Have a force measurement application or need for stress analysis on your aircraft project? Then contact the experts at Micro-Measurements to explore solutions using strain gage technology.

By Eric Stewart. Contributing Editor, Kitplanes
Photos: Eric Stewart

Several months before taking the Abaris class, I was able to take a similar continuing-education type engineering class offered by Micro-Measurements, a subsidiary of Vishay Precision Group that specializes in foil strain gauge sensors.

Those of you who attended the Reno Air Races may have seen the TECAT Performance Systems table set up in one of the sport class hangars. TECAT is a wireless torque measuring system that is moving from automotive applications (namely manufacturer testing and auto racing) into aviation by offering a system that provides in-flight crankshaft torque (and thus, with rpm input, horsepower to a cockpit interface). This to me looks like the way of things to come, and I introduced a few people to TECAT just because I thought it was super interesting. Well, next thing I knew, TECAT had sold a couple of units and needed someone to do on-site aviation installs, and asked if I’d like to help them out. This required that I take the Micro-Measurements class to become familiar with strain gauges and their installation (the TECAT system uses M-M strain gauges). As with Abaris, I was able to get the class fee waived in exchange for offering to write a review of the class. And again, I feel no conflict of interest here because the M-M course was, like the Abaris course, absolutely fantastic.

Strain gauge bonded to Lycoming crankshaft for TECAT installation.

The M-M student kit includes a large 3-ring binder with copies of all PowerPoint slides used in the class, a strain gauge calculator and quick lookup guide, tech notes, and tips. In addition, all the equipment for in-class gauge bonding is included—gauges, adhesive, soldering iron, etc.

The M-M student kit includes a large 3-ring binder with copies of all PowerPoint slides used in the class, a strain gauge calculator and quick lookup guide, tech notes, and tips. In addition, all the equipment for in-class gauge bonding is included—gauges, adhesive, soldering iron, etc. They pack a lot into a two-day course, and I could feel my brain stretching by the hour. Compared to the Abaris class, the Micro-Measurements class is even more theory intense, although this stands to reason since there are numerous factors that can throw off the gauge readings if not installed/compensated correctly, so a good basic understanding of how strain gauges work is essential for good installation and troubleshooting.

Bonding a gauge onto a composite surface (fiberglass PCB).

On the morning of our first day, we went over the theory and history of strain gauges and discussed how to select the appropriate strain gauge for a particular task, which adhesive to choose, and how to prepare the test surface to which the gauge will be bonded. We then started a lab project, bonding our first gauge to an aluminum coupon using a high-performance cyanoacrylate (similar to Super Glue) before stopping for lunch. After lunch we discussed instrumentation, followed by finishing the lab project (soldering on the wiring and checking/comparing our installations by cantilevering a weight from the end of the coupon and measuring strain with a data-acquisition unit). Because the aluminum coupons used have a known modulus of elasticity, comparing the readings given by our gauge installations revealed errors that could affect the installation (i.e., too much glue under a gauge, failure to properly remove soldering flux residue from the solder tabs, etc.). Before stopping for the day, we bonded a second gauge to a composite coupon of G10, otherwise known as breadboard or PCB (printed circuit board). The adhesive used for this project was a room-temperature-cure epoxy that needed an overnight cure.

Students have the opportunity to install gauges to both metal and composites.

On the morning of Day 2, we continued with the second project. We again hung a weight from the cantilevered coupon, checking the quality of our installation against a known value, and comparing/troubleshooting problems that appeared in anyone’s coupon. This was followed by continued discussion of instrumentation and how to correct for errors induced by temperature, etc. Although there is somewhat more theory in the M-M class, this makes sense because most of it is related to how various environmental factors can cause data errors, and is, thus, associated with the actual strain gauge installations. Due to the high sensitivity of these instruments, good installation procedures are a must.

Gauges are connected to a data acquisition unit to allow students to test the quality of their install and also illustrate factors that can lead to measurement error.

Although I think strain gauges are super cool, and there is no doubt that instrumenting your airplane could provide you with some interesting insights, I’ve featured the M-M class not because I think the average KITPLANES® reader is particularly interested in strain gauges, but because I think this type of class can really add another dimension to your homebuilding experience. Even if you only go to an EAA course at Oshkosh, do go.

This article was originally published in Kitplanes magazine, August 2018. It is presented here, with permission from Eric Stewart

Training of customers in the proper use of strain measurement techniques is an essential part of the Micro-Measurements philosophy. In support of this principle, Micro-Measurements conducts an extensive series of regularly scheduled technical seminars, workshops, and short courses.

These training programs are intended for managers, design and test engineers, technicians, teachers, and anyone else involved in stress analysis testing or in transducer development and manufacture.

Although most of the training sessions are conducted at Micro-Measurements facilities in the United States and Europe, many are also held at hotels and educational institutions around the world.